Past exam of the mathematics course of the University of Cambridge 2001 iii Paper 32 3 ii Solution Created 2026-10-03 Updated 2026-10-14
For the unit-scale Laplace distribution, integrating separately over the two half-lines gives the moment-generating functionfor ; the appropriate half-line integral diverges outside that interval. ThereforeIts derivatives needed for the saddlepoint density approximation areSet . Solving and selecting the root in gives the stable formulaincluding at . The other quadratic root is outside the domain. At the correct saddlepoint,Substitution gives the saddlepoint density of a Laplace sample mean:It is symmetric in , as the underlying Laplace distribution requires. At zero its value is , consistent with the local normal approximation having variance . This is an approximation, not an exact finite- density or an automatically normalized one.